Only a few proteins are synthesized abundantly by Drosophila tissue culture cells after heat shock. This and other evidence suggests that only a few regions of the genome are transcribed under these conditions. We propose to isolate individual species of heterogeneous nuclear RNA and messenger RNA from Drosophila tissue culture cells following heat shock. In situ hybridization with polytene chromosomes will be used as an assay to guide the purifications. If a single molecular species of high molecular weight nuclear RNA and the corresponding messenger RNA can be isolated, we will compare their secondary and nucleotide sequence structures.